Looking Up: Sundials, an age-old link to the sky

Friday

Jun 18, 2010 at 12:01 AMJun 18, 2010 at 6:12 PM

This time of year gardeners are happily occupied tending their flowers, whether in the backyard, the deck or even on a window sill if that’s all the space you have. No, this column hasn’t switched subjects from the stars! Not when we can talk about garden sundials and what they have to do with the sky above!

Peter Becker

This time of year gardeners are happily occupied tending their flowers, whether in the backyard, the deck or even on a window sill if that’s all the space you have. No, this column hasn’t switched subjects from the stars! Not when we can talk about garden sundials and what they have to do with the sky above!

As most everyone knows, a sundial is a way to tell the time by the moving sun; a shadow is cast from a stick called a gnomon onto a dial giving the hours. While of course you can just have a sundial for a decorative object and not care how good it is to tell time, to use it right takes some knowledge of astronomy.

Remember, one of the 100 billion stars or so in the Milky Way galaxy (I’m still counting; I’ll let you know when I’m finished) shines on us every day.

The sun’s powerful light is owed by it’s close proximity, only 93,000,000 or so miles away. No other star casts a shadow on Earth! If they did, we’d have Betelgeuse-dials or North Star-dials or Vega-dials to give us the time at night.

Using the sun, the time it gives will be slightly off from your wrist watch (unless of course your watch isn’t working, then it is exactly right twice every day when it matches the sundial’s or Dipper’s reading!).

The sundial’s gnomon, basically a straight-edged rod, has to be aligned to true north, pointing at the North Celestial Pole. This is the exact spot, very close to the North Star, about which the entire sky, day or night, appears to rotate (yes, believe it or not, the North Star is still there in broad daylight; you just can’t see it). The gnomon should also be tilted at the right angle, corresponding to your latitude above the equator. If it is off, you can still get a rough reading of the time, as long as it is not TOO far off.

Even so, the time won’t always match your watch. Noon hour on the sundial shows when the sun actually crosses the meridian, the imaginary line extending due south, straight up and down again to the North Celestial Pole in the northern sky.

Noon won’t always be at the same moment for various reasons, including the fact that the Earth varies in its distance from the sun.

When the Earth is closest to the sun (which occurs in January), the sun seems to travel slightly faster in the sky than when the Earth is back farther as it is in July.

We refer to the sundial’s measurement as Apparent Solar Time. Your wrist watch is regulated to show the average apparent solar days over the year and is referred to as the Mean Solar Time (”mean” as in “average”). There is still another problem. The time we set our watch is quite approximate, since our time zone is quite broad geographically (depending on your latitude; it is most broad at the equator and narrows to nothing at either the North or South pole).

Your watch time is based on the center of the time zone, which, when compared to the sun’s position, will vary by about a half hour from a time zone border to the center, at mid-northern latitudes.

But don’t let all this befuddle your enjoyment of the sundial! They make great lawn and garden décor and are reminders of a time (no pun intended) when people really relied on the sun, as well as other stars and moon, for everyday living. Pity we have become so technical that we have nearly forgotten our connection with the heavens!